The present invention relates to automatic equalizers which compensate for the distorting effects of band-limited channels on transmitted data signals.
Automatic equalizers are necessary for accurate reception of high-speed data signals transmitted over band-limited channels with unknown transmission characteristics. The equalizer is generally in the form of a transversal filter in which a sampled signal comprised of samples of an analog data signal are multiplied by respective tap coefficients. The resulting products are added together to generate an equalizer output which is then demodulated and/or quantized to recover the transmitted data. In addition, an error signal is formed equal to the difference between the equalizer output and a reference signal which represents the transmitted data symbol. The value of the symbol that was transmitted may be known at the receiver a priori, as is the case in many equalizer start-up arrangements. Alternatively, as in the so-called adaptive type of automatic equalizer, the reference signal is derived from the decision made in the receiver (on the basis of the equalized signal value) as to what data symbol was transmitted. In either case, the error signal is used to update the tap coefficient values in such a way as to minimize a measure of the distortion--primarily intersymbol interferences--introduced by the channel. The most commonly used error-directed coefficient updating algorithm is the so-called mean-squared error algorithm, which adjusts the tap coefficients so as to minimize the average of the value of the square of the error signal.
Most high-speed data receivers incorporate a synchronous, or baud, equalizer in which the analog data signal is sampled at a rate equal to the symbol rate. It is, however, possible to use a so-called fractionally spaced equalizer in which the analog data signal is sampled at a higher rate. Data decisions, i.e., quantizations of the equalizer outputs, are still made at the symbol rate. However, the fact that equalization is carried out using a finer sampling interval provides the fractionally spaced equalizer with significant advantages over its more conventional cousin. Most notable among these is insensitivity to channel delay distortion, including sampling phase errors.
There is, however, at least one significant problem unique to the fractionally spaced equalizer. In a synchronous equalizer, one set of tap coefficients is clearly optimum, i.e., provides the smallest average mean-squared error. By contrast, many sets of coefficient values provide approximately the same average mean-squared error in the fractionally spaced equalizer. As a consequence of this property, the presence of small biases in the coefficient updating processing hardware--such as biases associated with signal value round off--can cause at least some of the coefficient values to drift to very large levels, or "blow up," even though the average mean-squared error remains at, or close to, its minimum value. The registers used to store the coefficients or other signals generated during normal equalizer operation can then overflow, causing severe degradation, or total collapse, of the system response.
The copending U.S. patent application of J. J. Werner, Ser. No. 213,463, filed Dec. 5, 1980, which is a continuation of U.S. patent application Ser. No. 84,857, filed Oct. 15, 1979, now abandoned, teaches that the above-outlined coefficient drift problem can be controlled by causing the sampled signal to have energy in frequency bands in which the sampled channel transfer function has substantially zero gain, those frequency bands being referred to as "no-energy bands." This is illustratively achieved by adding to the analog data signal an out-of-band analog signal having energy in at least one no-energy band to form a composite signal which is then sampled.